1. Field of the Invention
The present invention is related to a movement detection apparatus suitably applicable to remote sensing by a TV camera, a moving image compression apparatus, a vibration-proof camera for correcting a vibratory motion of camera, etc.
2. Related Background Art
There are various methods and means for detecting movement of camera or subject. An example of movement vector detection apparatus using an image is described in U.S. Pat. No. 3,890,462.
That Patent discloses a method in which a luminance difference or interframe difference is obtained at a pixel from two sequential images, a space gradient is computed in a frame, and a movement amount is gained by division of the gradients per block.
The following formulae explain the above operation, where .alpha. stands for a movement amount in the x-direction and .beta. for that in the y-direction. ##EQU1##
In the formulae, b represents an operation block size and g(F, x, y) an image. A character F denotes a frame number and d a difference between two frames or a interframe difference. Space gradients are defined by .differential.g/.differential.x=g.sub.x ' and .differential.g/.differential.y=g.sub.y '.
This movement vector detection method has a problem in that the detection range of the movement amount is small, causing an extremely big detection error upon detection of a great movement amount. That is, a quick motion cannot be detected using a TV signal of a fixed frame rate.
FIG. 1 shows a simulation result of the movement detection operation applying the conventional method to an actual image.
The simulation assumptions are that a screen is composed of 512.times.512 pixels, and that a striped black and white pattern of 32 pixel period is photographed by a lens diameter of ten pixel circle of least confusion. A block size for the operation is 25 pixels.
As seen in FIG. 1, accurate detection of the movement amount is restricted within the actual movement of four to five pixels. Beyond the five pixel movement, the detection accuracy decreases so that detection values get away from the ideal characteristics. Moreover, in case that the actual movement amount is over eight pixels, the detection value becomes reduced. For example, at the twelve pixel actual movement, the detection value is about four pixels, which cannot be distinguished from the actual movement of four pixels.
Since the detection value becomes smaller upon greater actual movement, this apparatus shows a further greater detection error when calculating an acceleration or a difference per unit time between two movement amounts.
The detection range may be normalized by the pattern period .lambda. of a subject. An accurately detectable range is between .lambda./4 and .lambda./6, presenting a theoretical limit of performance of the conventional method.
Below are listed prior applications and patents of the present applicant concerning apparatus detecting movement like in the present invention.
______________________________________ Application or Patent Application date ______________________________________ U.S. Pat. No. 4788596 Japanese Patent Appln. Laid-open No. 1-178916 U.S. Ser. No. 880152 6/30/96 U.S. Ser. No. 154078 2/9/88 U.S. Pat. Nos. 5031049; 5,198,896; 4,939,685; 5,012,270; 5,107,293; 5,164,835; and 5,128,768 ______________________________________